Link quality agent

ABSTRACT

A link quality agent running in a computer embedded in a device, such as an optical communication transceiver, to monitor an optical signal quality of the link. In one embodiment, avalanche photodiode (APD) and filter wheel values on each side of the free space link as well as packet errors from a distribution switch are monitored. When the link quality agent determines that the free space link is marginal (based on the monitored optical signal quality), the link quality agent configures down the port between the distribution switch and the router on the receive end of the free space link, and forces customer traffic over a backup link. When both ends of the free space link are again functional, the link quality agent configures the port up and routes customer traffic to the free space link.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of application Ser. No.09/782,956, filed Feb. 13, 2001 now U.S. Pat. No. 6,678,251 B2, thebenefit of the priority date of which is claimed under 35 U.S.C. § 120.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to free space optical communicationsystems and, in particular, to an intelligent agent in a free spaceoptical communication system.

2. Background Information

Optical wireless transmission is way of providing multi-gigabitconnectivity between two locations without the use of an optical fiberinterconnection. At the transmitting end, an optical wireless systemtypically consists of an optical laser source, which is being modulatedby a data source to produce and optically encoded data signal. The datasource typically provides the intelligence or information to betransmitted, such as data, audio, video, messages, etc. An opticalamplifier may amplify the optical signal, which is then transmitted intothe atmosphere (or free-space) through a transmitting telescope as anoptical transmission beam towards a receiving telescope. On thereceiving end of an optical wireless system, a receiving telescopecollects part of the optical beam and focuses it as a light spot into areceiving optical fiber. The receiving optical fiber is connected to areceiver/regenerator, which converts the optically encoded data signalback into an electrical data signal.

In an optical wireless system, the atmosphere is the propagation mediumfor the optical transmission beam (sometimes called a light beam). Onedrawback to using the atmosphere as the transmission medium is theeffect that weather conditions have on the optically encoded datasignal. For example, fog often causes the optical link to operatemarginally, which means that the receiver may receive sufficient smallpacket handshaking signals, such as open shortest path first (OSPF)“hello” packets, but not enough of the transmitted data in largepackets.

Normally, when data is not being received, the system would shift to abackup means to deliver data. In the situation in which the system ismarginal, however, the system may not shift to a backup means to deliverthe data and much of the data is lost. Alternatively, the system mayshift to a backup means when the receiver does not receives sufficient“hello” packets and return to the primary means to deliver data when thereceiver does receive sufficient “hello” packets. The term “flapping”describes the condition in which the optical link goes in and out oftolerance numerous times such that the router has to excessivelyreconfigure the means to deliver data. Flapping consumes a largepercentage of bandwidth and therefore must be controlled. Moreover, inthis situation, the system experiences high link error rates, whichresults in the system having to retransmit packets. Retransmission ofpackets seriously degrades link bandwidth for data.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood by reference to the figures whereinreferences with like reference numbers generally indicate identical,functionally similar, and/or structurally similar elements. The drawingin which an element first appears is indicated by the leftmost digit(s)in the reference number in which:

FIG. 1 is a high-level block diagram of a communication system accordingto an embodiment of the present invention;

FIG. 2 is a more detailed block diagram of the communication system inFIG. 1; and

FIG. 3 is a flowchart illustrating an approach to link qualitymonitoring.

DETAILED DESCRIPTION

FIG. 1 is a high-level block diagram of a communication system 100. Inthe following description, numerous specific details, such as particularprocesses, materials, devices, and so forth, are presented to provide athorough understanding of embodiments of the invention. One skilled inthe relevant art will recognize, however, that the invention can bepracticed without one or more of the specific details, or with othermethods, components, etc. In other instances, well-known structures oroperations are not shown or described in detail to avoid obscuringaspects of various embodiments of the invention.

Some parts of the description will be presented using terms such asagent, link, light beam, transceiver, photon, remote monitors, and soforth. These terms are commonly employed by those skilled in the art toconvey the substance of their work to others skilled in the art.

Other parts of the description will be presented in terms of operationsperformed by a computer system, using terms such as receiving,detecting, collecting, transmitting, and so forth. As is well understoodby those skilled in the art, these quantities and operations take theform of electrical, magnetic, or optical signals capable of beingstored, transferred, combined, and otherwise manipulated throughmechanical and electrical components of a computer system; and the term“computer system” includes general purpose as well as special purposedata processing machines, systems, and the like, that are standalone,adjunct or embedded.

Various operations will be described as multiple discrete stepsperformed in turn in a manner that is most helpful in understanding theinvention. However, the order in which they are described should not beconstrued to imply that these operations are necessarily order dependentor that the operations be performed in the order in which the steps arepresented.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, process, step,or characteristic described in connection with the embodiment isincluded in at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

The communication system 100 can be any communication system thatemploys an analog transmission medium or that operates in an analogencoding environment. The communication system 100 may be an opticalsystem that uses the atmosphere as the transmission medium and in whichdata is transmitted and received via an optical carrier. In otherembodiments, the communication system 100 is an any wirelesscommunication system where the atmosphere is the transmission medium anddata is transmitted and received across an analog link via a radiofrequency (RF) carrier, a microwave carrier, and the like.

The system 100 as illustrated in FIG. 1 includes a transceiver 102 and atransceiver 104, which to transmit and receive traffic in free space viaan antenna 106 and an antenna 108, respectively. Traffic can be customertraffic or test traffic, which are small bursts of data.

In one embodiment, the transceiver 102 may be a single accesstransceiver (SAT) and the transceiver 104 may be a holographic photoncollector (HPC). Either the transceiver 102 or 104 may be a reflectivephoton collector (RPC), a multiple access transceiver (MAT), an SAT, anHPC, or any other device to transmit and receive a light beam from freespace.

Transceivers typically have analog signal strength detectors. One suchanalog signal strength detector is an avalanche photodiode (APD), whichis a photon detector. Transceivers also may employ techniques toattenuate the light beam, such as filter wheels. In operation, if thephoton count for the light beam at the detector is low, the light beamattenuator can be adjusted to allow more photons to be received.

The antennas 106 and 108 can be telescopes. Transceivers and antennassuitable for implementing the transceiver 102 ad 104 and the antennas106 and 108 are well known.

The transceiver 102 is coupled to a network device, such as adistribution switch 110, which filters and forwards packets. In oneembodiment, the distribution switch 110 allows multiple Ethernetconnections. Network devices are well known and distribution switchessuitable for implementing the distribution switch 110 are well known.The distribution switch 110 is coupled to a network router 112 and toone or more transceivers 113.

The transceivers 113 are typically similar to the transceiver 102 andthe transceiver 104, which are well known.

The network router 112 moves packets of data from source to destination,typically by passing messages and analyzing a routing table to determinethe best path for packets to take. The network router 112 alsoconfigures the best route for the packets to reach their ultimatedestination. Network routers suitable for implementing the networkrouter 112 are well known. The network router 112 is coupled to anetwork backbone 114.

The network backbone 114 generally is a connection of multipoint hubs(MPH), customer premised equipment (CPE), and/or points of presence(POP). The multipoint hubs may be connected to each other via reflectivephoton collectors (RPC). In an embodiment, the network of multipointhubs and reflective photon collectors is the network backbone 114.Backbones suitable for implementing the network backbone 114 are wellknown.

The transceiver 104 is coupled to a network device, such as adistribution switch 120, which may be similar or identical to thedistribution switch 110. The distribution switch 120 is coupled to anetwork router 122.

The network router 122 in an embodiment is similar to or identical tothe network router 112. The network router 122 is coupled to a router124 in a customer network 126. The router 124 also is similar to oridentical to the network router 112. The customer network 126 in anembodiment is typically any well-known local area network (LAN) or widearea network (WAN), including topology, protocol, and architecture,which is well known.

The network router 122 also is coupled to an alternate channel, such asa backup channel 128. In one embodiment, the backup channel 128 is a T-1line, which is a well-known dedicated phone connection supporting datarates of 1.544 Megabits per second (Mbps).

An agent 152 and an agent 154 monitor the performance of the opticallink through the atmosphere, and may access equipment's state variables(e.g., antenna, transceiver, router, computer, or remote device). Todetermine whether to route traffic through the backup channel 128, theagents 152 and 154 may employ a computational module, a neural network,or any suitable decision-making technique.

According to an embodiment of the present invention, the agents 152 and154 are a collection of software modules associated with thetransceivers. In one embodiment, the agents 152 and 154 are running on adevice embedded in the transceivers. The agents 152 and 154 monitoroptical information, such as optical detector values and light beamattenuator values for each transceiver. In one embodiment, the agents152 and 154 determine the quality of the analog signal by monitoring thephoton count using an APD and the light beam attenuation using a filterwheel, for example. There may be other ways to determine the quality ofthe analog signal besides monitoring the photon count using an APD andthe light beam attenuation using a filter wheel. For example, when thedata is transmitted and received via a radio frequency (RF) carrier asquelch reading on a radio may be used.

The agents 152 and 154 also monitor router information, such as numberof packets, number of packet errors, and so forth, accessed from thedistribution switches 110 and 120, respectively. The agents 152 and 154may access optical and router information using a variety of techniques.For example, the agents 152 and 154 may access the distribution switchesusing a well-known Simple Network Management Protocol (SNMP) interface.The agents 152 and 154 may access routers using a well-known commandline interface (CLI) or well-known remote monitoring (RMON) networkmanagement protocol.

The agents 152 and 154 are invoked when the quality of the opticalsignal or the router information is marginal. In the embodiment shown inFIG. 1, when the agents 152 and 154 are invoked data, such as customertraffic, is rerouted from the free space transmission medium to thebackup channel 128. When the quality of the optical signal or the routerinformation returns to normal, data, such as customer traffic, is routedback to the free space transmission medium from the backup channel 128.

A digital signal processor (DSP) 170 and a DSP 172 each control trackingof the light beam for the transceivers 102 and 104, respectively. In oneembodiment, the DSP 170 and a DSP 172 positions the transceivers 102 and104 to ensure the light beam is properly focused.

FIG. 2 shows the system 100 in more detail, showing state diagrams ofthe agents 152 and 154. The agent 152 includes a link quality linkmodule LQMLink 202, a link quality performance module LQMPerf 204, alink quality server module LQMServ 206, and a link quality configurationmodule LQMConfig 208. For every link quality server module there is alink quality performance module.

The LQMLink 202 accesses the distribution switch 110 to monitor routerinformation (e.g., number of packets, number of packet errors, and soforth). The LQMLink 202 also accesses log files, such as log files 210,which store optical information, to keep track of photon counts andfilter wheel settings, for example.

The LQMLink 202 launches LQMPerf 204 and LQMServ 206. The link qualityperformance module LQMPerf 204 and the link quality server moduleLQMServ 206 are generated by “Netperf”, which is a well-known networkperformance tester. Other network performance testers may be used togenerate the link quality performance module LQMPerf 204 and the linkquality server module LQMServ 206.

The LQMConfig 208 accesses a configuration file Config File 211. TheConfig File 211 configures a link to run an agent in the future. Asdepicted in FIG. 2, there may be a software application interfacebetween the agent 152 and the Config File 211.

The agent 154 includes a link quality link module LQMLink 212, a linkquality performance module LQMPerf 214, a link quality server moduleLQMServ 216, a link quality configuration module LQMConfig 218, a linkquality proxy module LQMTouch 220, and a link quality control moduleLQMCntrl 222.

The LQMLink 212 performs substantially the same or identical function inthe transceiver 104 as the LQMLink 202 does in the transceiver 102. Inone embodiment, the LQMLink 212 accesses the distribution switch 120 andlog files 230. The log files 230 performs substantially the same as oridentical function in the log files 210. As depicted in FIG. 2, theremay be a software interface, such as a Common Application SoftwareInterface, between the agent 154 and the Config File 232.

The LQMPerf 214 performs substantially the same as or identical functionin the transceiver 104 as the LQMPerf 204 does in the transceiver 102and the LQMServ 216 performs substantially the same as or identicalfunction in the transceiver 104 as the LQMServ 206 does in thetransceiver 102. The LQMPerf 204 communicates with the LQMPerf 214 toexchange test traffic. The LQMServ 206 communicates with the LQMServ 216to exchange test traffic. The link quality performance module LQMPerf214 and the link quality server module LQMServ 216 are generated by“Netperf”, which is a well-known network performance tester. Othernetwork performance testers may be used to generate the link qualityperformance module LQMPerf 214 and the link quality server moduleLQMServ 216.

The LQMConfig 218 performs substantially the same as or identicalfunction in the transceiver 104 as the LQMConfig 208 does in thetransceiver 102. In one embodiment, the LQMConfig 218 accesses aconfiguration file Config File 232, to test the configuration of theanalog (e.g., radio wave, microwave, laser, light, or the like withinthe electromagnetic spectrum) link and to configure one or more links torun an agent in the future.

The LQMTouch 220 is a proxy for the LQMLink 202 in the transceiver 104.In one embodiment, the LQMTouch 220 opens a socket in the transceiver104 and listens to the optical and router information on behalf of theLQMLink 202. The LQMLink 202 connects to the LQMTouch 220.

The LQMLink 202 or the LQMTouch 220 writes a file(s) to indicate thepreferred status of the transceiver. For example, the LQMLink 202 maywrite an “up local” file to indicate that the transceiver 102 should beup and operating, a “down local” file to indicate that the transceiver102 should be down for customer traffic and operating only for testtraffic, an “up remote” file to indicate that the transceiver 104 shouldbe up and operating, or a “down remote” file to indicate that thetransceiver 104 should be down for customer traffic and operating onlyfor test traffic. Likewise, the LQMTouch 222 may write an “up local”file to indicate that the transceiver 104 should be up and operating, a“down local” file to indicate that the transceiver 104 should be downfor customer traffic and operating only for test traffic, an “up remote”file to indicate that the transceiver 102 should be up and operating, ora “down remote” file to indicate that the transceiver 102 should be downfor customer traffic and operating only for test traffic.

The LQMCntrl 222 monitors and acts on the existence of the files.

FIG. 3 is a flowchart 300 depicting an approach to link qualitymonitoring. In step 302, the LQMLink 202, the LQMLink 212, the LQMCntrl222, and the LQMTouch 220 are initiated. In one embodiment, Sysmon,which is a well-known network monitoring tool designed to provide highperformance and accurate network monitoring, initiates the LQMLink 202,the LQMLink 212, the LQMCntrl 222, and the LQMTouch 220.

In step 304, customer traffic is transmitted from transceiver totransceiver via the primary channel (free space). The LQMLink 202 andthe LQMLink 212, analyze and integrate the (digital) router informationwith the (analog) optical information to decide whether to routecustomer traffic through the backup channel 128. If the photon countfalls below a threshold value, the LQMLink 202 and the LQMLink 212interpret this as marginal optical link performance. If the photon countis above a threshold value, the LQMLink 202 and the LQMLink 212 allowsthe customer traffic to flow through the primary channel (via theoptical link).

In step 306, the LQMLink 202 or the LQMLink 212 detect that the photoncount is below a threshold value. The LQMLink 202 or the LQMLink 212also may detect that the number of packet errors exceeds a thresholdvalue. The LQMLink 202 or the LQMLink 212 also may detect a sudden spikein the number of packet errors or a sustained increase in the ratio ofpacket errors to the number of packets transmitted when computed over aparameterized number of samples. The LQMLink 202 or the LQMLink 212write a “down local” file or a “down remote” file for either transceiver102 or 104, respectively. Of course, the link agents 152 and 154 may useother techniques other than packet error values and the ratio of packeterrors to the number of packets transmitted to monitor the performanceof the optical link through the atmosphere to determine whether to routecustomer traffic through the backup channel 128.

In step 308, the LQMCntrl 222 detects a “down local” file or a “downremote” file for either transceiver 102 or 104, the LQMCntrl 222 telnetsto the network router 122 via the CLI interface to disable the portbetween the network router 122 and the distribution switch 120. Thenetwork router 122 detects that the port between the network router 122and the distribution switch 120 is disabled and performs a well-knownrouting technique, such as OSPF, and finds another route for customertraffic, such as through the backup channel 128 to the network backbone114.

When the port between the network router 122 and the distribution switch120 has been disabled and customer traffic has been rerouted, in step310, the LQMLink 202 and/or the LQMLink 212 launch the LQMServ 206 orthe LQMServ 216, respectively, and begin sending test traffic via theLQMPerf 204 and the LQMPerf 214, respectively, to test the optical link.The LQMPerf 204 communicates with the LQMServ 206 and the LQMPerf 214communicates with the LQMServ 216 to send and receive the test trafficacross the optical link. The LQMLink 202 or the LQMLink 212 continue tomonitor the distribution switches 110 and 120, which will provide thenumber of packets and the number of packet errors for the test traffic.

This process is iterative and continues until the photon count is abovea threshold value and the ratio of packet errors to the number ofpackets transmitted is below a threshold value. In one embodiment, whenthe photon count is above a threshold value and the ratio of packeterrors to the number of packets transmitted is below a threshold valuethe agents 152 and 154 continue to hold the port between the networkrouter 122 and the distribution switch 120 disabled for a configurableperiod of time, to prevent flapping, for example. If the photon countfalls below the threshold value or the ratio of packet errors to thenumber of packets transmitted is above the threshold value, the countdown of the configurable period of time is reset.

In step 312, when the photon count returns to being above a thresholdvalue and the ratio of packet errors to the number of packetstransmitted returns to being below a threshold value for theconfigurable period of time, this is detected, the LQMLink 202 writes an“up local” file to indicate that the transceiver 102 can come up andreceive customer traffic and an “up remote” file to indicate that thetransceiver 104 can come up and receive customer traffic. The LQMTouch222 writes an “up local” file to indicate that the transceiver 104 cancome up and receive customer traffic and an “up remote” file to indicatethat the transceiver 102 can come up and receive customer traffic. TheLQMCntrl 222 detects the “up local” file and the “up remote” file forboth transceivers 102 or 104 and telnets to the network router 122 viathe CLI interface to enable the port between the network router 122 andthe distribution switch 120. When the port between the network router122 and the distribution switch 120 is enabled, customer traffic flowsthrough the port between the network router 122 and the distributionswitch 120 and out to free space, which is the primary channel.

If the LQMCntrl 222 detects a “down local” file or a “down remote” filefor either transceiver 102 or 104 after the port between the networkrouter 122 and the distribution switch 120 has been recently disabled,the LQMCntrl 222 telnets the network router 122 via the CLI interface todisable the port between the network router 122 and the distributionswitch 120 and the configurable period of time may be reconfigured to alonger period of time. This can be used to prevent flapping as well.

Although embodiments of the present invention are described with respectto an optical free-space communication system, the present invention isnot so limited. For example, the present invention may be implemented inany system that transmits and receives using an analog transmissionmedium or that operates in an analog encoding environment.

1. A system comprising first and second wireless optical systemtransceivers to exchange customer traffic via a primary channelcomprising a wireless optical system link; first and second networkdevices coupled to the first and second wireless optical systemtransceivers, respectively, to selectively route the customer trafficvia the primary channel or via an alternate channel; and first andsecond link quality agents, coupled to the first and second wirelessoptical system transceivers, respectively, and coupled to the first andsecond network devices, respectively, to monitor an optical signalquality of the wireless optical system link and to control the first andsecond network devices to route the customer traffic to the alternatechannel and to route test traffic to the wireless optical system linkwhen the optical signal quality of the wireless optical system link isdetermined by at least one of the first and second link quality agentsto have entered a marginal state, wherein the first and second linkquality agents monitor one of an analog or a digital quality of thewireless optical system link.
 2. The system of claim 1, the first andsecond link quality agents further to reroute the customer traffic backto the wireless optical system link via control of the first and secondnetwork devices when it is determined by at least one of the first andsecond agents that the optical signal quality of the wireless opticalsystem link has returned to a non-marginal state.
 3. The system of claim1, wherein the alternate channel routes traffic via a computer networkcoupled between the first and second network devices.
 4. The system ofclaim 1, wherein the alternate channel employs a different transportmedium than the wireless optical system link.
 5. A method, comprising:initiating a link quality agent; transmitting customer data betweenfirst and second wireless optical system transceivers over a primarychannel comprising a wireless optical system link; monitoring an opticalsignal quality of the wireless optical system link via the link qualityagent; and rerouting the customer data to an alternate channel andtransmitting test data over the wireless optical system link if the linkquality agent determines the quality of the wireless optical system linkis marginal, wherein said wireless optical system link and the alternatechannel route the customer data along different transport mediums. 6.The method of claim 5, wherein the optical signal quality of thewireless optical system link is determined to have entered a marginalstate by determining a received analog signal strength is below athreshold value.
 7. The method of claim 5, wherein the optical signalquality of the wireless optical system link is determined to haveentered a marginal state by determining that a received packet errorcount is above a threshold value.
 8. The method of claim 5, wherein theoptical signal quality of the wireless optical system link to haveentered a marginal state by determining that a ratio of packet errors toa number of packets received is above a threshold value when computedover a parameterized number of samples.
 9. The method of claim 5,wherein the optical signal quality of the wireless optical system linkis determined to have entered a marginal state by determining that aratio of packet errors to a number of packets received is above athreshold value and that a received analog signal strength is below athreshold value when computed over a parameterized number of samples.10. The method of claim 5, further comprising rerouting the customerdata to be transmitted over the wireless optical system link anddiscontinuing transmission of the test data over the wireless opticalsystem link when it is determined by the link quality agent that theoptical signal quality of the wireless optical system link has returnedto a non-marginal state.
 11. The method of claim 10, wherein reroutingthe customer data from the wireless optical system link to the alternatechannel and rerouting the customer data back to the wireless opticalsystem link comprise respective switchover conditions, furthercomprising implementing a configurable delay between when the quality ofthe link is determined to have changed between marginal and non-marginalstates and when an associated changeover condition occurs to preventnetwork flapping.
 12. The method of claim 5, wherein the quality of thewireless optical system link is determined to have entered a marginalstate by determining that both an analog quality of the link and adigital quality of the link have fallen below a threshold level ofperformance.
 13. The method of claim 5, wherein the wireless opticalsystem link employs a first transport medium and the alternate channelemploys a second transport medium different from the first transportmedium.
 14. A machine-readable medium having machine-readableinstructions stored thereon, which when executed cause a machine toperform the operations of: initiating a link quality agent; transmittingcustomer data between first and second wireless optical systemtransceivers over a primary channel comprising a wireless optical systemlink; monitoring an optical signal quality of the wireless opticalsystem link via the link quality agent; and rerouting the customer datato an alternate channel and transmitting test data over the wirelessoptical system link if the link quality agent determines the opticalsignal quality of the link is marginal, wherein said wireless opticalsystem link and the alternate channel route the customer data alongdifferent transport mediums.
 15. The machine-readable medium of claim14, wherein execution of the machine instructions further performs theoperation of monitoring a detector output to determine if a receivedanalog signal strength is below a threshold value, whereby the opticalsignal quality of the link is determined to have entered a marginalstate.
 16. The machine-readable medium of claim 14, wherein execution ofthe machine instructions determines that the optical signal quality ofthe link has entered a marginal state by performing the operation ofdetermining that a received packet error count is above a thresholdvalue.
 17. The machine-readable medium of claim 14, wherein execution ofthe machine instructions further performs the operations of determiningif the optical signal quality of the wireless optical system link hasreturned to a non-marginal state, and in response thereto rerouting thecustomer data to be transmitted over the wireless optical system linkand discontinuing transmission of the test data over the wirelessoptical system link.
 18. The machine-readable medium of claim 14,wherein rerouting the customer data from the wireless optical systemlink to the alternate channel and rerouting the customer data back tothe wireless optical system link comprise respective switchoverconditions, and wherein execution of the machine instructions furtherperforms the operation of implementing a configurable delay between whenthe quality of the link is determined to have changed between marginaland non-marginal states and when an associated changeover conditionoccurs to prevent network flapping.
 19. The machine-readable medium ofclaim 14, wherein the wireless optical system link employs a firsttransport medium and the alternate channel employs a second transportmedium different from the first transport medium.
 20. A method forsending customer data from a first location to a second location,comprising: (a) providing a primary channel comprising a wirelessoptical system link enabled by respective wireless optical systemtransceivers disposed at the first and second locations; (b) providingaccess to a backup channel comprising a network connection between thefirst and second locations; (c) routing customer data over the primarychannel while monitoring an optical signal quality of the wirelessoptical system link to determine if the link enters a marginal operatingstate, and in response thereto, (d) rerouting the customer data over thebackup channel; and (e) sending test data over the wireless opticalsystem link while monitoring the optical signal quality to determine ifthe link returns to a non-marginal operating state, and in responsethereto, (f) rerouting the customer data back to the primary channel,wherein rerouting the customer data from the primary channel to thebackup channel and rerouting the customer data back to the primarychannel comprise respective switchover conditions, the method furthercomprising implementing a configurable delay between when the linkquality is determined to have changed between marginal and non-marginalstates and when an associated changeover condition occurs to preventnetwork flapping; and (g) repeating operations (c)–(f) on a continuousbasis.